1. Field of the Invention
The present invention relates generally to a method of manufacturing Group III nitride crystals, a method of manufacturing a semiconductor substrate, Group III nitride crystals, a semiconductor substrate, and an electronic device.
2. Related Background Art
A Group III nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a “Group III nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor devices that emit green, blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of, for instance, biotechnology and an ultraviolet LED as, for example, an ultraviolet source for a fluorescent lamp.
Substrates of a Group III nitride semiconductor (for example, GaN) that are used for LDs or LEDs usually are formed by heteroepitaxially growing Group III nitride crystals on a sapphire substrate using vapor phase epitaxy. Generally, crystals obtained by this method have a dislocation density of 108 cm−2 to 109 cm−2 and thus reducing the dislocation density has been an important issue for this method. In order to resolve this issue, how to reduce the dislocation density has been considered, and, for example, an epitaxial lateral overgrowth (ELOG) method has been developed (see, for instance, JP11(1999)-145516A). With this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2, but the manufacturing process is complicated.
On the other hand, besides the vapor phase epitaxy, a method of growing crystals from a liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of Group III nitride single crystals such as GaN, conventionally it has been understood that conditions including 8000 atm (8000×1.013×105 Pa) and 1200° C. are required for growing GaN from a liquid phase. In this connection, recently, it was disclosed that the use of a Na flux allowed GaN to be synthesized at a relatively low temperature and pressure, specifically, 750° C. and 50 atm (50×1.013×105 Pa) (see, for instance, U.S. Pat. No. 5,868,837).
Recently, single crystals whose maximum crystal size is about 1.2 mm are obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm (50×1.013×105 Pa), and then crystals are grown for 96 hours using the melt (see, for instance, JP2002-293696A).
Furthermore, another method also has been reported in which a GaN crystal layer is formed on a sapphire substrate by a metalorganic chemical vapor deposition (MOCVD) method and then single crystals are grown by a liquid phase epitaxy (LPE) method (Jpn. J. Appl. Phys., Vol. 42, (2003) pp4-6).
However, the quality of the Group III nitride crystals obtained using the conventional techniques may not be sufficient. Hence, there are demands for a technique of manufacturing crystals of higher quality.